1. Field of the Invention
The present invention generally relates to a continuous heat treating furnace and more particularly, to a heat-exchanging type continuous heat treating furnace equipped with an automatic flow control system in which energy-saving measures are fully conducted.
2. Description of the Prior Art
Conventionally, in a continuous heat treating furnace having a pre-heating zone, a high-temperature treating zone and a cooling zone, various attempts have been proposed to promote energy-saving.
Japanese Patent Publication (Tokkosho) No. 45-10610 discloses one of the continuous heat treating furnaces which is arranged substantially in the form of a figure "U" so that the pre-heating and cooling zones may be disposed in a parallel relationship to each other, with a partition wall spaced from a ceiling wall being interposed between both zones. Such an arrangement enables heat-exchange between the pre-heating and cooling zones.
The continuous heat treating furnace of the above described type is, however, disadvantageous in that since atmosphere gas in one zone i.e., in the pre-heating or the cooling zone is mixed therein, a large temperature difference between the atmosphere gas and a workpiece to be treated can not be obtained and accordingly, heat-exchanging efficiency is undesirably low.
U.S. Pat. No. 4,449,923 (corresponding to Japanese Patent Application No. 56-13126) and Japanese Patent Application No. 56-182515, both proposed by the same applicant of the present invention, were developed to eliminate the above described disadvantage.
In the former, the pre-heating and cooling zones are united into one and a charge end portion of the pre-heating zone and a discharge end portion of the cooling zone are communicated with each other through a recirculation duct provided outside the furnace so that high-temperature atmosphere gas contained in the high-temperature treating zone may be caused to pass through the pre-heating zone to pre-heat the workpiece to be treated while the atmosphere gas which has been lowered in temperature through the heatexchange with the workpiece is circulated back to the hightemperature treating zone.
Even in this kind of the furnace, since the temperature of the atmosphere gas is substantially leveled throughout the pre-heating and cooling zones, an average temperature difference between the atmosphere gas and the workpiece is relatively small. As a result, it is disadvantageously difficult not only to render an end temperature of the workpiece by the pre-heating or by the cooling to a desired one, but also to keep a predetermined cooling rate with respect to the workpiece.
The latter is of a construction such that both of the pre-heating and cooling zones are separated into a plurality of compartments each accommodating a convection fan on its ceiling portion while a terminal compartment of the pre-heating zone is communicated with a first compartment of the cooling zone through a duct so that the atmosphere gas in the cooling zone may be drawn into the terminal compartment of the pre-heating zone and further towards a first compartment thereof.
Hereupon, a flow rate of the atmosphere gas flowing from the pre-heating zone towards the cooling zone or from the cooling zone towards the pre-heating zone is inversely proportional to an absolute temperature of the atmosphere gas in the zone from which the atmosphere gas is supplied, that is, the absolute temperature of the atmosphere gas in the pre-heating zone in the case where the atmosphere gas is supplied from the pre-heating zone towards the cooling zone or that in the cooling zone in the case where the atmosphere gas is supplied from the cooling zone towards the pre-heating zone. In particular, since the temperature of the atmosphere gas in the pre-heating zone varies greatly according to the presence of the workpiece to be treated, the flow rate of the atmosphere gas to be supplied from the pre-heating zone towards the cooling zone also greatly varies.
The terms "flow rate" used hereinbefore and hereinafter are to be understood as meaning of a gas volume flowing in a unit period under a standard state at 0.degree. C. and 1 atm.
In either of the above described constructions, since the flow rate of the atmosphere gas supplied through the duct changes, the atmosphere gas in the pre-heating zone or in the cooling zone is introduced into the heating zone. Consequently, the atmosphere gas drawn into the heating zone is unnecessarily uneconomically heated therein.